I'm surprised the game even runs at all when doing this.
I can't make any promises about it because I never tried to make a game work with all renderers, but, maybe this will work with the next demo build. (And that's a big maybe).

The characters (and other things) uses "instance uniforms" in their shaders, and these don't exist in the OpenGL renderer, so the shader compilation fails and nothing appears on screen.
Except, support for this was added to Godot back in October, and the next demo is running with the latest version of the engine. So assuming this is the only issue (probably not), it might work.

Do you even get a proper framerate with an integrated gpu? If you do, it might be worth it to add a compatibility mode to the game so it runs on older hardware. I only have my computer and a Steamdeck, so I never tried to run it on anything else.